The present invention relates to the field of composite reinforced metal type materials, in which a reinforcing material is compounded with matrix metal to form a so called two phase or reinforced material. In such reinforced material, the reinforcing material may be in the form of fibers, threads, whiskers, powder, or the like; and the material of this reinforcing material may be boron, carbon, alumina, silica, silicon carbide, carbon, ceramic, or the like, or mixtures thereof, which have high strength and high elasticity. Further, as matrix metal may be used a metal such as aluminum or magnesium or an alloy thereof.
In motor vehicles and aircraft and so forth, nowadays, the constant demand for lightening and strengthening of structural members and parts has meant that construction from light materials such as aluminum or magnesium has become common. Problems arise, however, in making parts from aluminum or magnesium or their alloys, despite the light weight of these materials, and despite their easy workability, because the mechanical characteristics of such materials such as strength, including bending resistance, torsion resistance, tensile strength, and so on are inferior to those of competing materials such as steel. Further, the occurrence of cracking and the spreading of cracks in parts made of aluminum or magnesium or alloys thereof can be troublesome. Therefore, for parts the strength of which is critical there are limits to the application of aluminum or magnesium or their alloys.
Accordingly, for such critical members, it has become known and practiced for them to be formed out of so called two phase or composite materials, in which reinforcing material is dispersed within a matrix of metal. Thus, if the matrix metal is aluminum or magnesium alloy, then the advantages with regard to weight and workability of using this type of alloy as a constructional material can be obtained to a large degree, while avoiding many of the disadvantages with regard to low strength and crackability; in fact, the structural strength of the composite materials made in this way can be very good, and the presence of the reinforcing material can stop the propagation of cracks through the aluminum or magnesium alloy matrix metal.
Various proposals have been made with regard to compositions for such fiber reinforced metal type composite materials, and with regard to methods of manufacture thereof and apparatuses for performing such manufacture. However, one of the best so far implemented has been the high pressure casting method, a summary of which, as far as its conventional practice is concerned, will now be given.
First a mass of reinforcing material such as reinforcing fibers or the like is placed in the mold cavity of a casting mold, and then a quantity of molten matrix metal is poured into the mold cavity. The free surface of the molten matrix metal is then pressurized to a high pressure such as approximately 1000 kg/cm.sup.2 by a plunger or the like, which may be slidingly fitted into the mold. Thereby the molten matrix metal is intimately infiltrated into the interstices of the mass of reinforcing material, under the influence of this pressure. This pressurized state is maintained until the matrix metal has completely solidified. Then finally, after the matrix metal has solidified and cooled into a block, this block is removed from the casting mold, and the surplus matrix metal around the reinforcing material is removed by machining, so that the composite material mass itself, consisting of the mass of reinforcing material impregnated with matrix metal, is isolated. This high pressure casting method has the advantage of low cost, and it is possible thereby to manufacture an element of a relatively complicated shape with high efficiency.
With regard to this high pressure casting method, as is described in Japanese patent application Ser. No. Sho 55-107040 (1980), which is a patent application by the same applicant as the applicant of the parent Japanese patent application Ser. No. Sho 57-207219 of the present patent application of which priority is being claimed in the present application, the reinforcing material mass may be preheated to a substantially high temperature of at least the melting point of the matrix metal, before the matrix metal is poured into the mold cavity of the casting mold, in order to aid with the proper penetration into and proper impregnation of the reinforcing material by the matrix metal. This preheating ensures that as the molten matrix metal infiltrates into the interstices of the reinforcing material, it is not undesirably cooled down by the reinforcing material being cold, so as to at least partly solidify. Such solidification, if it occurs, much deteriorates the impregnation of the reinforcing material by the matrix metal, and accordingly this type of preheating is very beneficial. More details will be found in the above identified Japanese patent application or laying open publication, if required.
Further, as is described in Japanese patent application Ser. No. Sho 56-32289 (1981), which is also a patent application by the same applicant as the applicant of the parent Japanese patent application Ser. No. Sho 57-207219 of the present patent application, the reinforcing material mass may be, before the casting process, charged into a case (which may be made of stainless steel or the like) of which only one end is left open, an air chamber being left between the reinforcing material mass and the closed end of the case, and then the case with the reinforcing material mass therein may be placed into the mold cavity of the casting mold, and pressure casting as described above may be carried out. This concept of utilizing a case with an air chamber being left therein again serves to aid with the proper penetration into and proper impregnation of the reinforcing material by the matrix metal, because the air left in the air chamber, when the matrix metal is pressurized at the outside of the case, will be compressed to almost nothing as the matrix metal in the molten state flows through the interstices of the reinforcing material in a directed fashion towards the air chamber, and thereby the proper penetration of the matrix metal into the interstices of the reinforcing materials is very much helped. More details will be found in the above identified Japanese patent application or laying open publication, if required.
Now, with regard to the per se conventional preheating discussed above, in this high pressure casting method, this is conventionally done by heating up the reinforcing material, which typically has been formed into a shaped mass, to said substantially high temperature at least equal to the melting point of the matrix metal, and then by rapidly putting the reinforcing material into the mold cavity of the casting mold and immediately rapidly pouring the molten matrix metal into said mold cavity around the reinforcing material, shortly subsequently applying pressure to infiltrate said molten matrix metal into the reinforcing material. However, when this is done, the following difficulties arise.
First, if the reinforcing material shaped mass is much smaller in size than the mold cavity of the casting mold, in which case said shaped mass may be supported within the mold cavity upon supports as suggested in the previously identified Japanese patent application, then the advantage is obtained that no substantial loss of heat occurs from the thus preheated reinforcing material to the sides of the mold cavity, before the matrix metal in the molten state has been completely poured into said mold cavity. On the other hand, the disadvantage is caused that the finished composite material mass produced consists of a mass of reinforcing material infiltrated with matrix metal with a relatively thick layer of solidified pure matrix metal around it. Now, this is often very inconvenient for post processing of the composite material, since stripping off of such a thick layer of matrix metal from the outside of the finished produced metallic block part of which is composite reinforced by the included reinforcing material is substantially troublesome, and since in many applications a piece of material is required which is substantially completely composed of reinforced material, i.e. is without any parts made only of matrix metal.
But if instead the reinforcing material shaped mass is almost equal in size to the mold cavity of the casting mold, then the disadvantage is caused that substantial loss of heat occurs from the thus preheated reinforcing material to the sides of the mold cavity, before the matrix metal in the molten state has been completely poured into said mold cavity, which can seriously deteriorate the infiltration of the molten matrix metal into the interstices of the reinforcing material and the quality of the resulting composite material; although on the other hand the advantage is obtained that the finished composite material mass produced consists of a mass of reinforcing material infiltrated with matrix metal with a relatively thin layer of solidified pure matrix metal around it, which as explained above is often very convenient for post processing of the composite material. In fact, it has in the prior art appeared quite difficult to resolve this conflict.
Further, when the reinforcing material mass is, before the casting process, charged into a case of which only one end is left open, an air chamber being left between the reinforcing material mass and the closed end of the case, and the high pressure casting process is carried out with the reinforcing material remaining in this case, as outlined above and as described in the previously identified Japanese patent application at length, then, although the proper penetration into and proper impregnation of the reinforcing material by the matrix metal is thereby greatly aided, the problems described above with regard to isolating the finished composite material, which of course involved stripping off of the case from the outside of the finished composite reinforced material, are intensified. In the event that the case is made of stainless steel, which is a suitable material therefor, the difficulty of removing the finished composite material from the case is so high as to be unacceptable in practice. This has, in the prior art, made it difficult to take advantage of the above described prior art concept of including the reinforcing material in a case while forming the composite material.